How The Ph Of Bromothymol Blue Affects The Absorbance Value

Research Question: To investigate how varying the pH of bromothymol blue affects the absorbance value of the solution which determines the equilibrium constant (pKa) of the indicator.

Variables:

Variables Variables Measured Method of measurement
Independent

pH of the six buffer solution A pH probe attached to a data-logger will be used to measure pH
Dependent

Absorption of the buffer solutions at wavelength 435.0nm and 617.0 nm A spectrophotometer (±0.001) will be used to measure absorbance at wavelength 436.0nm and 617.0 nm respectively

Hypothesis: Since Bromothymol blue is a weak acidic compound, it will have a pKa value around 7 under an acidic region.

To proceed with this investigation, the absorbance of the six buffer solutions at the two wavelengths will be plotted and the final graph of absorbance against pH will be similar to Figure 1 below:
Figure 1: The graph of absorbance of (HIn) and (In- ) against pH.
The red line indicates the changes of absorbance at wavelength 1 where the acidic form (HIn), absorbs strongly. The blue line indicates the behavior at wavelength 2 where the basic form (In-), absorbs strongly.

As mentioned, it is evident that the pH where (HIn) = (In-) corresponds to the indicators pKa. Thus, the pKa of an indicator corresponds to the pH of the solution at the inflection point in a plot of absorbance as a function of pH, as indicated by the dotted line in figure 1.

Hence, the ratio of [〖In〗^- ]/[HIn] at any pH can be obtained using the equation below:

[〖In〗^- ]/[HIn] =〖A_λ2∙A〗_(λ1,acidic )/〖A_(λ2,basic)∙A〗_(λ1
)

Using this equation, the pKa can then be determined by plotting the graph of pH against log [〖In〗^- ]/[HIn] , illustrated in figure 2 below:

Figure 2: Relationship between pH and log (In-)/(HIn) with the pKa of the indicator corresponding to the intersection of the line with the pH axis where log[〖In〗^- ]/[HIn] =0
Apparatus:

Chemicals Apparatus (Quantity)
Bromothymol blue Spectrometer (1)

Mol Sodium Hydroxide
(NaOH) 50 cm3 Volumetric flask (6) Mol Sodium Phosphate Dibasic
(Na2B4O7 . 10H2O) 100cm3 beaker (6)

Burette (2)

Mol of Citric Acid
(C6H8O7) pH probe (1)

Data logger (1)

0.025 Mol Sodium Tetraborate Decahydrate (Na2HPO4) Plastic cuvettes (6)

Dropper (2)

Distilled Water